A bicycle derailleur, a rear derailleur for instance, has a chain guide which rotatably supports a guide pulley and a tension pulley. This chain guide is pivotally supported via a shift link mechanism such as a parallelogram pantograph link mechanism while being elastically urged in a direction to tension a chain. When a control lever, which is connected by a control cable to the shift link mechanism, is operated, the shift link mechanism is deformed to cause the chain guide, which is supported by a movable member of the shift link mechanism, to move axially of a hub shaft, forcing the chain to move to a selected sprocket of a multiple free wheel.
For the above-mentioned control cable, there is commonly employed what is known as a pull type cable wherein pulling force can only be transmitted. Therefore, the derailleur is provided with a return spring which elastically urges the shift link mechanism constantly toward the returning direction. Hence, when the control lever is turned to pull the control cable, the shift link mechanism is forced to deform in a direction where the elastic force of the return spring is accumulated. When, on the other hand, the control lever is turned otherwise to release the control cable, the accumulated force of the return spring causes the shift link mechanism to return to the original position. This movement of the shift link mechanism causes the movable member which supports the chain guide to perform a reciprocal movement corresponding to the reciprocal turning movement of the control lever.
The return spring is mounted conventionally around a pin which pivotally connects two adjacent members of the shift link mechanism. This return spring is mounted in such a way that ends of the spring elastically contact the respective constituent members of the shift link mechanism.
With this arrangement, the return spring will be contracted by an amount corresponding to a relative pivotal movement of the two members between which the ends of the return spring are placed: For instance, when these two members make a relative pivotal movement of 45.degree., the spring is also contracted by about 45.degree.. According to the Hooke's law, the return spring's elastic force is proportional to its torsional deformation. This means that if the return spring receives a great amount of contraction such as described hereinabove within the shift link's deformation range, the difference in the spring's elastic force between beginning and ending phases of the shift link's deformation will also be great, posing a problem of deteriorated speed shift operatability.
To elaborate on this point, in order for the speed shift control lever to be retained at a selected rotational position, the control lever must be given a predetermined amount of friction resistance (or click resistance.) The amount of this resistance must be determined according to a maximum elastic force of the return spring. This means that as the elastic force of the return spring increases, a greater friction resistance must be provided, which results in a greater rotational resistance of the control lever with deteriorated operatability thereof. In addition, it calls for an increase in the size of a friction generating mechanism and the entire control lever assembly.
As a means to solve the above-mentioned problem, the difference in elastic force caused by the difference of the return spring's contraction may be made smaller by increasing the number of windings of the coil spring, or in other words, by increasing an overall length of the spring wire. This solution, however, will result in a very large coil spring to be used, with a decreased productivity in assembly process and an increased size and weight of the derailleur mounted with this coil spring.